The objective of this program is to study on a molecular basis the nature of the control of genetic activity. It is proposed to study such control on three levels, namely, the material nature of repressors, the nature of the unit genetic switching act by which the gene is repressed or derepressed, and on the level of the genetic switching network, the way in which individual genetic switching units are linked together to bring about sequential developmental processes. It is proposed to attempt to achieve the desired objective by the use of a variety of methods; the study of the histones and other protein components of chromatin, physical studies of the interaction of repressor and DNA, chemical and physical studies of the basis of specificity between repressor and gene, physical studies of the structure of chromatin and particularly of its nucleohistone component, as well as physical and biochemical studies of the interaction of small molecules such as hormones with chromatin from appropriate target organs. It is proposed, too, to use in our attempts to achieve the desired objective, a variety of kinds of systems--the cellular slime mold, Drosophila, sea urchin, Xenopus and mammalian--using for each study the chromosomal system which appears most suited to the particular objective.